The present invention relates to units for inertial measurement of movement, more specifically although not exclusively intended to be incorporated into laser gyro inertial systems activated by mechanical vibrations and intended to be fitted in aircraft.
These inertial measurement units have, when mounted on moving bodies, to be protected by dampers from the parasitic mechanical influences of the vibration or jolt type so that such influences do not disrupt their operation and cause the least possible amount of noise in their measurement signals; although, to a certain extent, it is possible to combat the noise that affects the measurement signals of an inertial system as a result of parasitic mechanical influences using digital processing, this is costly in terms of computation power which means that every attempt is made to reduce the parasitic mechanical influences as far as possible.
To achieve this, it is commonplace for the sensors of an inertial measurement unit: gyros and accelerometers, to be mounted on or in a support chassis which is both rigid and lightweight, suspended inside a casing via a small number of elastic damping pads.
When the gyros are laser gyros, a further problem arises as a result of the use, for combating their lack of sensitivity at low rotational speeds, of a mechanical activation which, by reaction, gives rise to parasitic movements of the casing.
Specifically, the principle of a single-axis laser gyro is based on the use of two monochromatic light beams propagating in opposite directions in a plane along the same path in a closed loop. When the plane of the path of the two counter-propagating monochromatic light beams is rotated with respect to its normal, the effective lengths of the paths travelled by the two beams change, leading to a difference in frequency between the two beams because the frequency of oscillation of a laser is dependent upon the length of the path travelled by its beam. This difference in frequency, which can be measured by making the two beams interfere on a photodetector, then gives a measurement of the speed at which the plane of the path of the two light beams is rotating about its normal, which is the sensitive axis of the gyro. However, when the difference in path length of the two beams is small, the two light beams couple and oscillate at one and the same frequency, which means that it is impossible to measure low rotational speeds. To alleviate this lack of sensitivity of single-axis laser gyros at low rotational speeds, it is known practice for them to be given an oscillatory vibratory mechanical movement about their sensitive axis so as to increase their apparent speed of rotation and allow small speeds of rotation to be measured. The offset on the rotational measurement produced by this auxiliary mechanical vibratory movement is eliminated later by appropriate processing of the signal supplied by the gyro.
Three-axis laser gyros consist of an assembly, in a rigid block, of three single-axis laser gyros with their sensitive axes oriented along the three axes of a trihedron involving three right angles and, possibly, common elements. To overcome their lack of sensitivity at low rotational speeds, they are activated by a single oscillatory vibratory mechanical movement about an axis which is oriented differently from their sensitive axes, in a direction such that this movement has oscillatory components of like amplitude about the three sensitive axes of the three single-axis laser gyros.
The vibratory mechanical activating movement is obtained, in general, by mounting the rigid block consisting of the three three-axis laser gyros, known as the three-axis laser gyro assembly between two support rings which are spaced out along and centred on its axis of activation and have a certain degree of flexibility in terms of torsion and in terms of rotation. These support rings are attached rigidly by their hub or interior wall, to the three-axis gyro assembly. They act as insert pieces which, via their rim, or exterior periphery, contact a cylindrical sleeve which surrounds the three-axis laser gyro assembly and acts as a chassis supporting both the three-axis laser gyro assembly and the accelerometer sensors of the inertial measurement unit. The two support rings are fixed by their rim or exterior periphery to the interior wall of the cylindrical sleeve. One of them is fitted with a piezoelectric oscillatory motor for generating and sustaining the activating rotary oscillations. The cylindrical sleeve containing the three-axis laser gyro assembly and supporting the accelerometers is itself placed in a casing and attached to the interior wall thereof via a number of elastic damping pads spread uniformly about its periphery.
Of the parasitic mechanical influences to which laser gyros are sensitive, the most troublesome are those which give rise to conical movements of the gyros, combining rotations about directions transverse to their sensitive axes. Some of these conical movements originate from external disturbances transmitted to the laser gyro assembly by the casing, the suspension consisting of the elastic damping pads and the support rings which are flexible in terms of torsion and in terms of rotation; they manifest themselves more particularly at the frequency of the suspension, which is a low frequency of the order of some tens of hertz. However, others have their origins in the reaction of the casing to the mechanical activating movement transmitted to the laser gyro assembly via the suspension and the support rings. These manifest themselves at the activating frequency which is of the order of several hundreds of hertz.
To combat noise in the signals delivered by the gyros and which originate from the parasitic conical movements, it is commonplace for error compensation, compensating for the errors due to the conical movements, to be introduced by detecting these movements, estimating the influence they have on the gyro signals and digitally processing the gyro signals to compensate for these influences.
However, the parasitic conical movements at the activating frequency cannot be estimated and compensated for correctly by the customary digital processing operations because their frequency is too high. Attempts could be made at pushing back the frequency limit on compensatory digital processing operations, but this would entail the use of analogue/digital converters and a processing unit which perform far better than those currently used.
Another approach would be to take as many steps as possible to attenuate these parasitic conical movements at the activating frequency which are the root cause of troublesome residual noise and of very-low-frequency drift in the signals from the gyros. As these are transmitted to the gyros through their damped suspension, this is one possible line of approach. However, it would seem that the current way that gyros are suspended in their casings by elastic damping pads is not optimum because of the dissymmetry in the damping characteristics about the axis of activation which are caused by inevitable differences in stiffness between the elastic damping pads and which are the root cause of couplings between axes that encourage the parasitic conical movements to be transmitted to the gyro assembly.
The object of the present invention is to reduce the parasitic conical movements to which laser gyros with mechanical activating movement are subjected, in order to reduce the residual noise and the very-low-frequency drift that affects their signals, and to do so by improving the symmetry of the suspension of a gyro with respect to its axis of activation in order to reduce the amplitude of the transverse movements through coupling of the various degrees of freedom.
The subject of the present invention is an antivibration elastic suspension designed for attaching the chassis of an inertial measurement unit to the inside of a casing, a gyro assembly with a mechanical activating movement being fixed into the said chassis via two support rings which are flexible in terms of torsion and in terms of rotation, spaced along the chassis and centred along the axis of activation of the gyro assembly. This antivibration elastic suspension is noteworthy in that it comprises at least one flexible annular diaphragm arranged so that it is coaxial with the axis of activation of the gyro assembly and fixed by its interior periphery to the exterior periphery of the chassis and by its exterior periphery to the interior wall of the casing.
Advantageously, the said flexible annular diaphragm is a flat diaphragm.
Advantageously, the said flexible annular diaphragm is fixed to the exterior periphery of the chassis of the inertial measurement unit by trapping its interior edge between a fixing ring secured to the exterior periphery of the chassis and a clamping washer attached to the said fixing ring.
Advantageously, the said flexible annular diaphragm is fixed inside a rigid frame secured to the casing by trapping its exterior edge between the said frame and a clamping washer attached to the said rigid frame.
Advantageously, the said flexible annular diaphragm is made of a moulded flexible material, reinforced at its interior and exterior edges by two rigid hoops incorporated into the mass of the moulded material and pierced with screw-fastening orifices.
Advantageously, the said flexible annular diaphragm is made of a flexible plastic.
Advantageously, the said flexible annular diaphragm is made of a rubbery material.
Advantageously, this antivibration elastic suspension comprises two flexible annular diaphragms centred on the axis of activation of the laser gyro assembly and arranged so that they are spaced out along the height of the exterior wall of the chassis, each of these two flexible annular diaphragms being fixed by its interior edge to the exterior wall of the chassis and by its exterior edge to the interior wall of the casing.
Advantageously, the casing of the inertial measurement unit is cylindrical and comprises, on the inside, near its two ends, annular plates to which the flexible annular diaphragms of the antivibration elastic suspension are fixed.
Advantageously, the laser gyro assembly is fixed, in the inertial measurement unit, inside a cylindrical chassis, by means of at least one support ring which is placed at a top of the cylindrical chassis, and the exterior periphery of which is accessible from outside the cylindrical chassis and equipped with attachment means allowing the interior edge of a flexible annular diaphragm of the antivibration elastic suspension to be fixed to it.
This elastic suspension using diaphragms is far better at respecting symmetry with respect to the axis of activation of the laser gyro than an elastic suspension using a small number of isolated damping shoes. It thus makes it possible to reduce the couplings of axes to a great extent and therefore to reduce the amplitudes of the transverse parasitic movements caused by the reaction of the casing to the activating oscillations.